Metallocene catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. Metallocenes are now used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems with different substitution patterns.
These metallocenes can be used in solution polymerisation but results of such polymerisations have generally been poor. These metallocenes are therefore conventionally supported on a carrier such as silica. Research has found that heterogeneous catalysis gives rise to better polymer products than homogeneous catalysis (in solution). The use therefore of a support is common place. Despite several years of development of this catalyst technology, there is still room for improved activity of course.
In WO03/051934, the inventors proposed an alternative form of catalyst which is provided in solid form but does not require a conventional external carrier material such as silica. The invention is based on the finding that a homogeneous catalyst system containing an organometallic compound of a transition metal can be converted, in a controlled way, to solid, uniform catalyst particles by first forming a liquid/liquid emulsion system, which comprises as the dispersed phase, said solution of the homogeneous catalyst system, and as the continuous phase a solvent immiscible therewith, and then solidifying said dispersed droplets to form solid particles comprising the said catalyst.
The invention described in WO03/051934 enabled the formation of solid spherical catalyst particles of said organotransition metal catalyst without using e.g. external porous carrier particles, such as silica, normally required in the art. Thus, problems relating to catalyst silica residues can be solved by this type of catalyst. Further, it could be seen that catalyst particles having improved morphology, will give, due to the replica effect, polymer particles having improved morphology as well.
Although a lot of work has been done in the field of metallocene catalysts, both with conventional supported catalysts as well with solid catalysts prepared according to the principles as described in said WO03/051934, there still remain some problems, which relate especially to the productivity or activity of the catalysts. There remains a need therefore to improve the productivity or activity of the catalysts. Furthermore, there is a need to obtain propylene polymers, having particular properties, such as higher melt temperature (Tm). In addition there is a need to keep the activity or productivity at a desired high level and at the same time produce polymers having higher Tm.
There remains a need therefore to find new catalysts for olefin polymerisation, which are able to produce these polymers with desired properties, especially high melting points (Tm) and which have high activity and/or productivity. Further, it is highly desired in many polymer applications that inorganic residues, e.g. silica residues, in the final product are reduced as much as possible.
As a consequence, the inventors set out to develop a process and a catalyst for that process having a superior polymerisation in terms of higher activity for propylene polymerisation, leading to propylene polymers having higher melting points.
The present inventors have now found that certain olefin polymerisation catalysts are able to solve the problems disclosed above, and which catalysts are not previously described in the art. In particular, the invention combines the use of boron based and aluminoxane cocatalysts with metallocenes in solid catalysts, however, not containing any external support material, essentially prepared using the basic principles of WO03/05194.
The invention provides a process which exhibits remarkable increase in polypropylene melting point due to the use of the solid catalyst material, where no silica support material is used as defined herein. This avoids any problems relating to the use of the conventionally supported catalysts, such as silica supported catalysts.
Whilst both boron based and aluminoxane cocatalysts are well known in the art, they are typically used as alternatives. However, it is also known to use boron activators together with aluminoxanes in some circumstances.
EP-A-0574258 discloses use of boron compounds together with aluminoxanes in single site catalysts. The catalysts are homogeneous and they are used in homogeneous polymerisation where activity increase could be observed.
In J Macromol. Chem Phys, 199, 2409-2416 (1998), there is a disclosure of the use of constrained geometry metallocene type catalysts with both a methyl aluminoxane and trispentafluorophenyl boron activator. In the context of solution phase polyethylene polymerisation, the blend was found to increase catalyst activity.
In the literature, there are also other similar observations, that homogeneous catalyst activity (solution phase polymerisation) was improved by using boron modification, but when heterogeneous catalysis was tried, i.e. when catalysts were supported on silica, activity was lower than that achieved using MAO activators alone.
However, WO1998/140418 discloses that when specific types of boron-compounds, in particular alkyl or aryl boronic acids (RB(OR′)2) or cyclic boron compounds, boroxanes, are used with silica supported metallocene catalysts in combination with aluminoxanes, higher activity was seen for ethylene-butene polymerisation.
US2011294972 discloses the use of catalysts of specific transition metal complexes comprising mono-anionic, bidentate triazole ligands in combination with MAO and borate type activators supported on silica in ethylene-butene polymerisation.
In Macromol. Chem. Phys. 200, 2127-2135 (1999) page 2128, propylene polymerisation is discussed using a bridged biscyclopentadienyl type catalyst in the presence of both MAO and dimethylanilinium tetrakis(pentafluorophenyl) borate. When the metallocene is activated with MAO alone, there is a change in the polymer melting point (Tm). In this case, the presence of both MAO and the boron activator decreased the melting point which is the opposite of the desired goal in the present invention. It is surprising that the combination covered in the present invention allows an increase in melting point therefore.
The present inventors have surprisingly found that the use of both boron based and aluminoxane cocatalysts in combination with the use of a solid, but unsupported metallocene allows the formation of a catalyst which allows increases in the melting point of the propylene polymer formed relative to one produced with only one cocatalyst present. Moreover, this increase in melting point can be achieved at very high catalyst activities and can be accompanied by a reduction in MFR.